The evolution of steady-state laurdan fluorescence: From model membranes to cellular applications.

Laurdan (6-dodecanoyl-2-(dimethylamino)-naphthalene) is a solvatochromic fluorescent probe widely used for investigating membrane biophysical properties. Since its first application in phospholipid bilayers, laurdan has become a valuable tool in this field, owing to its sensitivity to the mobility and dynamics of surrounding lipid carbonyl groups and its ability to report on membrane phase behavior through the generalized polarization (GP) parameter. GP, a ratiometric empirical parameter, is derived from the ratio of emission intensities at approximately 440 and 490 nm, providing a measure of membrane fluidity ranging from rigid gel phases to more hydrated liquid-crystalline states. This review outlines the spectroscopic evolution of laurdan applications, beginning with steady-state fluorescence measurements in model membranes and progressing through more modern methodologies including anisotropy measurements, two-photon excitation microscopy, fluorescence correlation spectroscopy, and spectral phasor analysis. Key developments in laurdan's application to biological systems are discussed, including investigations of lipid raft-like domains, heavy metal-membrane interactions, and cellular membrane organization. Practical considerations for the exogenous incorporation of laurdan into membrane systems are also addressed, including the influence of solvent vehicle on probe aggregation and incorporation kinetics. Laurdan derivatives such as C-laurdan, CAPRYDAA, and organelle-targeted variants have further extended the versatility of this probe family. Together, these advances illustrate how laurdan has evolved from a simple polarity sensor into a multifaceted platform for characterizing membrane structure, lateral heterogeneity, and dynamics across model and biological systems.